Polyolefin films are used extensively for packaging and in film covering applications. The use of polyolefin films continues to increase as new market opportunities become available in areas where paper was traditionally used. The versatility of the film provides potentially infinite growth prospects for the product in the future. However, there is an inherent short coming in the use of plastic films that may retard its market acceptance and growth, it sticks. When plastic film is produced or used in various applications, there is a tendency for contacting layers of the film to stick together or “block”, making separation of the film, opening of bags made from the film, or finding the end of the film on plastic rolls difficult. The present invention relates to polyolefin resin compositions that are specifically designed to have satisfactory antiblocking capability.
Antiblock agents are materials that are added to polyolefin resins to roughen their surface and thereby prevent layers of the plastic film from sticking, hence the term, “antiblocking agent” is applied to such materials. Although, inorganic minerals, such as, for example, diatomaceous earth, synthetic silica and talc are known to reduce blocking when added to polyolefin film resin compositions, each has both advantages and critical disadvantages.
One comparative advantage of diatomaceous earth is that it is known to be a moderately effective antiblocking agent, when used as an antiblocking agent. However, it is also known that diatomaceous earth adversely affects the film's physical properties, such as film clarity, film haze, and is very abrasive and moderately expensive and may pose a serious health threat. Synthetic silicates are known to be effective as an antiblock, however a significant disadvantage of silica is that it is very expensive. Talc, on the other hand, has found increasing use as an effective antiblock agent over diatomaceous earth and synthetic silica because of a significant cost advantage over both. However, one major disadvantage when talc is added to polyolefin film resins, is that it aggressively adsorbs other film additives, such as antioxidants, slip agents and processing aid. The absence, or reduced level of these additives in polyolefin resin compositions during production, routinely cause processing problems and raise serious film quality concerns.
For example, antioxidants are added to improve film stability, slip agents are present in the resin to improve film converting, while processing aids are employed to improve film quality, and to provide lubrication during film extrusion by eliminating melt fracture. Melt fracture is a measure of film surface uniformity, appearance and strength. Of the three additives mentioned here, processing aids are most adversely affected by the presence of antiblock agents. Although it is well known that all antiblock agents adsorb processing aids, talc antiblock agents adsorb greater levels of processing aids than either diatomaceous earth or synthetic silica antiblocks. Consequently, when resin compositions are produced having additives that include antiblock talc, it is necessary to increase the dosage of processing aids. The increased dosage adversely effects the over all production economics of the plastic film.
Therefore, what is needed is a new generation of talc antiblock agents that adsorb less process aids than either synthetic silica or diatomaceous earth.